As the commercial market for magnetic recordings has flourished, the size and relative complexity of the facilities associated with duplicating such recordings has increased dramatically. Previously, limitations in the size of such facilities and the recording methods employed often made it unnecessary to deal with the electrical problems involved with interconnecting vast arrays of recording equipment, in some cases as many as tens of thousands of machines. However, the increased size of modern facilities, together with the improved quality of today's recording equipment, make it mandatory to ensure that the signals associated with such recording processes are kept as free as possible from any form of interference or distortion.
Whenever two pieces of electrical equipment are connected with conducting wires, it is possible that there will be a difference in the ground potential of the units, which can result in a flow of electrical current between the two ground potentials, typically referred to as a "ground-loop". With alternating current supplied at a frequency of 50 Hz or 60 Hz, these low frequencies, when manifested as an audio signal, are typically referred to as "hum." On a video screen, the result as may be seen as dark horizontal "hum bars" across the picture typically moving slowly from the bottom of the picture towards the top.
There are several methods for eliminating such undesirable effects in large systems, but these methods are based on cancellation of the interference, rather than prevention. In the case of audio signals, transformers or differential-input operational amplifiers are frequently used. Such approaches are effective if the signal distribution system employs a balanced-line wiring scheme, wherein the same signal is applied to two wires, with the phase of the signal on one of the wires being reversed. At the receiving end, a transformer or differential-input amplifier is employed to effectively combine the two signals, resulting in a signal with twice the amplitude. External signals appear as interference present on both wires, in equal amplitude, and having the same phase. As such, the resulting differential-input method leads to a cancellation of the interference signal. In addition, the two wires are typically carried in a shielded cable, in which the insulated signal wires are surrounded by a conductive-mesh or wrapped by a conductive-foil, thereby providing an effective electrostatic shield. To prevent the flow of any ground-loop currents, this shield is only connected at one end of the cable.
Unfortunately, video and other high frequency signals are typically distributed in an unbalanced, or "single-ended," wiring scheme, carried by coaxial cables. In this case, interconnection of two pieces of equipment may result in an electrical current flowing through the shield of the coaxial cable, producing the "hum" effects described above. In some cases, the connection of unbalanced wiring will even cause equipment using balanced wiring to exhibit hum, due to ground-loop effects from other wiring connections. The most common method of dealing with this problem involves the use of a "hum-stop" or "hum-canceling" coil, which consists of a coil of coaxial cable tightly wound to produce cancellation of the low frequency power components on the signal lines.
Such coils are expensive to manufacture with wide band frequency response, and invariably produce an attenuation of higher frequencies, due to the long length of cable contained in the coil. Compensation of this frequency loss requires extra circuitry, either as a pre-emphasis to the signal at the source end of the cable, or else as post-emphasis to the signal at the receiving end. In either case, there may be some degradation of the signal-to-noise ratio, and the additional circuitry may introduce non-linear distortions that can affect the differential phase or gain, or other characteristics, of the signal path. Occasionally, various remote control signals are also carried through unbalanced cables, and these may produce ground-loop problems as well. A carefully designed method of signal distribution which prevents these ground-loop problems in an economical way will be of benefit to the users of even small size duplication systems.